Girls, Ex girlfriend (dumper) acts passive aggressive towards me for no reason even though I was minding my own business, what gives?

Haven't seen each other in a year, she invites me to a party with mutual friends, we didn't speak to each other but there didn't seem to be any ill will. A few weeks later we went bowling (with mutual friends) but this time she was being hostile, giving me the silent treatment and overall just being unpleasent towards me.
Girls, Ex girlfriend (dumper) acts passive aggressive towards me for no reason even though I was minding my own business, what gives?
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